A file system like a linear tape file system (LTFS) is realized, for example as a mechanism which accesses the data on the tape drive as the file in the file system. In an the LTFS, the data partition for storing data and the index partition for storing meta-information (hereafter, although meta-information may also be called an “index”, it is the same meaning) are created. By reading meta-information from the index partition when loading the tape (when mounting the file system), the file system recognizes the data on the tape as files.
In the data partition, after data of a file has been written, as the next record, the meta-information of all the files that are present in the file system at that time point is written as an index. Before the tape is unloaded (the file system is un-mounted), the meta-information of all the files constituting the file system at that time is written as an index at the beginning of the index partition.
When the index is written on the data partition at frequent intervals, the proportion of the meta-information occupies on the data partition becomes too large and the size of data that can be written becomes too small. Therefore, as a compromise, when files have been written, the file system is defined (synchronized) by writing the index after five minutes have passed from the initial write operation. The index that is written is for all the files on the file system at that time point, and is not for the differences from the previous index that was changed. Therefore, if the number of files written increases even if the index is writing every five minutes, the size of the index will also increase in proportion to the number of files, and the file capacity in which can be written on the file system will decrease.